1. Field of the Invention
The present invention relates to the use of alkali metal borohydrides as blowing agents in connection with the production of foamed plastics.
2. Description of the Prior Art
The use of alkali metal borohydrides, such as sodium borohydride and potassium borohydride, as blowing agents in connection with the production of cellular articles is well known. For example, U.S. Pat. No. 2,951,819 utilizes an alkali metal borohydride in connection with the manufacture of cellular articles. U.S. Pat. No. 3,167,520 relates to cellular articles prepared from polystyrene by extruding polystyrene resin containing an alkali metal borohydride which decomposes during extrusion to form a cellular article.
U.S. Pat. Nos. 4,394,458 and 4,399,238, both assigned to the same assignee as the present application, disclose the use of alkali metal borohydrides such as sodium and potassium borohydride as blowing agents useful in connection with the production of articles having a foamed or cellular structure. One particularly advantageous embodiment of the inventions described therein involves the use of a stable borohydride concentrate mixture containing from about 0.5% to 20% by weight of an alkali metal borohydride with a dry inert resinous polymer. The concentrate typically contains no more than about 0.1% absorbed water or the borohydride will hydrolyze.
The concentrates described therein may be made by dry blending from 0.5 to 20% by weight of powdered alkali metal borohydride with a dry inert resinous polymer to form a mixture containing less than about 0.1% by weight water; heating the mixture to a temperature above the melting point of the resin to form a molten mixture; extruding the molten mixture and then cooling the extrusion to form a solid, stable concentrate; and then forming the cooled extruded concentrate into pellets by conventional techniques such as chopping, grinding and the like. Another aspect of the invention disclosed therein comprises a method for making cellular articles by contacting a mixture of alkali metal borohydride and a resinous polymer with a high surface area silica activating agent having a proton source adsorbed on its surface to hydrolyze the alkali metal borohydride, thereby causing the resinous polymer to have a foamed cellular structure. The alkali metal hydride therein may be in the form of the concentrate described therein or as neat powdered alkali metal borohydride.
The system described in the above patents can be described as a "two-part" system in that the resin to be foamed must be mixed with two components before extrusion. The first component is the alkali metal borohydride. The second component is the activator. Although the system described therein is fully functional and provides acceptable performance, it would also be desirable to supply the borohydride and activator as a single ingredient or mixture of ingredients, i.e. a one-part system, such that only one component need be measured out and mixed with the resin to be foamed prior to the extrusion step.